On the rainfall-altitude relationship in a mediterranean mountainous region : the South-East of France

Gilles Molinié (LTHE, Université de Grenoble, France); D. Ceresetti, J. D. Creutin, S. Anquetin and B. BoudevillainIt is usually admitted that rainfall amounts collected a year increase with raingauge altitudes. Using a 40-years long database of daily rainfall rates, this behavior of the rainfall with altitude is observed in the South-East of France. The mean rainfall depth collected a year at altitudes above 900 m is around 1.20 m while it is around 0.8 m for raingauges at altitudes below 200 m.

The raingauge network includes about 500 daily and 200 hourly sensors covering an area of about 150-200 km2. The database length is of 20 years at the hourly time scale. The question of a potential rainfall-altitude relationship is addressed for rainfall durations from the hour to the year. The rainfall rate is considered as the product of two random functions, the intermittency (probability of the rainfall rate above a given threshold) and the non-null rainfall intensity. The rainfall intermittency monotonously decreases with altitude. The intermittency pattern is similar whatever the season and the considered duration. The triggering of cloud-producing rainfall appears closely linked to the terrain elevation.

Concerning the non-null rainfall intensity, the altitude forcing is far more complex if it exists. It is strongly dependent on the duration, season, ... At the shortest duration (hour), the non-null rainfall intensity is uncorrelated with the raingauge altitude at some seasons and decreases with altitude at others. In increasing the duration, the part of rainfall intermittency incorporated into non-null rainfall intensities increases. Therefore, this impacts the behavoir of rainfall amounts with altitude at the longuest durations. The characteristic temporal scales of this process are described in the poster.